A number of liquefaction systems for cooling, liquefying, and optionally sub-cooling natural gas are well known in the art, such as the single mixed refrigerant (SMR) cycle, the propane-precooled mixed refrigerant (C3MR) cycle, the dual mixed refrigerant (DMR) cycle, C3MR-Nitrogen hybrid (such as AP-X™) cycles, the nitrogen or methane expander cycle, and cascade cycles. Typically, in such systems, natural gas is cooled, liquefied, and optionally sub-cooled by indirect heat exchange with one or more refrigerants. A variety of refrigerants might be employed, such as mixed refrigerants, pure components, two-phase refrigerants, gas phase refrigerants, etc. Mixed refrigerants (MR), which are a mixture of nitrogen, methane, ethane/ethylene, propane, butanes, and pentanes, have been used in many base-load liquefied natural gas (LNG) plants. The composition of the MR stream is typically optimized based on the feed gas composition and operating conditions.
The refrigerant is circulated in a refrigerant circuit that includes one or more heat exchangers and a refrigerant compression system. The refrigerant circuit may be closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled by indirect heat exchange in one or more refrigerant circuits by indirect heat exchanger with the refrigerants in the heat exchangers.
The refrigerant compression system includes a compression sequence for compressing and cooling the circulating refrigerant, and a driver assembly to provide the power needed to drive the compressors. The refrigerant compression system is a critical component of the liquefaction system because the refrigerant needs to be compressed to high pressure and cooled prior to expansion in order to produce a cold low pressure refrigerant stream that provides the heat duty necessary to cool, liquefy, and optionally sub-cool the natural gas.
Referring to FIG. 1, a typical DMR process of the prior art is shown in liquefaction system 100. A feed stream, which is preferably natural gas, is cleaned and dried by known methods in a pre-treatment section (not shown) to remove water, acid gases such as CO2 and H2S, and other contaminants such as mercury, resulting in a pre-treated feed stream 101. The pre-treated feed stream 101, which is essentially water free, is precooled in a precooling system 134 to produce precooled natural gas stream 102 and further cooled, liquefied, and/or sub-cooled in a main cryogenic heat exchanger (MCHE) 165 to produce LNG stream 104. The LNG stream 104 is typically let down in pressure by passing it through a valve or a turbine (not shown) and is then sent to LNG storage tank (not shown). Any flash vapor produced during the pressure letdown and/or boil-off in the tank may be used as fuel in the plant, recycled to feed, and/or sent to flare.
The pre-treated feed stream 101 is precooled to a temperature below 10 degrees Celsius, preferably below about 0 degrees Celsius, and more preferably below about −30 degrees Celsius. The precooled natural gas stream 102 is liquefied by cooling to a temperature between about −150 degrees Celsius and about −70 degrees Celsius, preferably between about −145 degrees Celsius and about −100 degrees Celsius, and subsequently sub-cooled to a temperature between about −170 degrees Celsius and about −120 degrees Celsius, preferably between about −170 degrees Celsius and about −140 degrees Celsius. MCHE 165 shown in FIG. 1 is a coil wound heat exchanger with two tube bundles, a warm bundle 166 and a cold bundle 167. However, any number of bundles and any exchanger type may be utilized.
The term “essentially water free” means that any residual water in the pre-treated feed stream 101 is present at a sufficiently low concentration to prevent operational issues associated with water freeze-out in the downstream cooling and liquefaction process. In the embodiments described herein, water concentration is preferably not more than 1.0 ppm and, more preferably between 0.1 ppm and 0.5 ppm.
The precooling refrigerant used in the DMR process is a mixed refrigerant (MR) referred to herein as warm mixed refrigerant (WMR), comprising components such as nitrogen, methane, ethane/ethylene, propane, butanes, and other hydrocarbon components. As illustrated in FIG. 1, a warm low pressure WMR stream 110 is withdrawn from the bottom of the shell side of precooling heat exchanger 160 and is compressed and cooled in WMR compression system 111 to produce compressed WMR stream 132. The WMR compression system 111 is described in FIG. 2. The compressed WMR stream 132 is cooled in a tube circuit of precooling heat exchanger 160 to produce a cold stream, which is then let down in pressure across first WMR expansion device 137 to produce expanded WMR stream 135. The expanded WMR stream 135 is injected into the shell-side of precooling heat exchanger 160 and warmed against the pre-treated feed stream 101 to produce the warm low pressure WMR stream 110. FIG. 1 shows a coil wound heat exchanger with a single tube bundle for the precooling heat exchanger 160, however any number of tube bundles and any type of heat exchanger may be employed.
In the DMR process, liquefaction and sub-cooling is performed by heat exchanging precooled natural gas against a second mixed refrigerant stream, referred to herein as cold mixed refrigerant (CMR).
A warm low pressure CMR stream 140 is withdrawn from the bottom of the shell side of the MCHE 165, sent through a suction drum (not shown) to separate out any liquids and the vapor stream is compressed in CMR compressor 141 to produce compressed CMR stream 142. The warm low pressure CMR stream 140 is typically withdrawn at a temperature at or near WMR precooling temperature and preferably less than about −30 degree Celsius and at a pressure of less than 10 bara (145 psia). The compressed CMR stream 142 is cooled in a CMR aftercooler 143 to produce a compressed cooled CMR stream 144. Additional phase separators, compressors, and aftercoolers may be present. The process of compressing and cooling the CMR after it is withdrawn from the bottom of the MCHE 165 is generally referred to herein as the CMR compression sequence.
The compressed cooled CMR stream 144 is then cooled against evaporating WMR in precooling system 134 to produce a precooled CMR stream 145, which may be fully condensed or two-phase depending on the precooling temperature and composition of the CMR stream. FIG. 1 shows an arrangement where the precooled CMR stream 145 is two-phase and is sent to a CMR phase separator 164 from which a CMR liquid (CMRL) stream 147 and a CMR vapor (CMRV) stream 146 are obtained, which are sent back to MCHE 165 to be further cooled. Liquid streams leaving phase separators are referred to in the industry as MRL and vapor streams leaving phase separators are referred to in the industry as MRV, even after they are subsequently liquefied.
Both the CMRL stream 147 and CMRV stream 146 are cooled, in two separate circuits of the MCHE 165. The CMRL stream 147 is cooled and partially liquefied in the warm bundle of the MCHE 165, resulting in a cold stream that is let down in pressure across CMRL expansion device 149 to produce an expanded CMRL stream 148, that is sent back to the shell-side of MCHE 165 to provide refrigeration required in the warm bundle 166. The CMRV stream 146 is cooled in the first and second tube bundles of MCHE 165, and reduced in pressure across the CMRV expansion device 151 to produce expanded CMRV stream 150 that is introduced to the MCHE 165 to provide refrigeration required in the cold bundle 167 and warm bundle 166.
MCHE 165 and precooling heat exchanger 160 can be any exchanger suitable for natural gas cooling and liquefaction such as a coil wound heat exchanger, plate and fin heat exchanger or a shell and tube heat exchanger. Coil wound heat exchangers are the state of art exchangers for natural gas liquefaction and include at least one tube bundle comprising a plurality of spiral wound tubes for flowing process and warm refrigerant streams and a shell space for flowing a cold refrigerant stream.
FIG. 2 shows the details of the WMR compression system 211. Any liquid present in warm low pressure WMR stream 210 is removed by passing through a phase separator (not shown) and the vapor stream from the phase separator is compressed in low pressure WMR compressor 212 to produce medium pressure WMR stream 213 that is cooled in low pressure WMR aftercooler 214 to produce cooled medium pressure WMR stream 215. The low pressure WMR aftercooler 214 may further comprise multiple heat exchangers such as a desuperheater and a condenser. The cooled medium pressure WMR stream 215 may be two-phase and sent to WMR phase separator 216 to produce a WMR vapor (WMRV) stream 217 and WMR liquid (WMRL) stream 218. The WMRV stream 217 is compressed in high pressure WMR compressor 221 to produce high pressure WMR stream 222 and cooled in high pressure WMR desuperheater 223 to produce desuperheated high pressure WMR stream 224. The WMRL stream 218 is pumped to produce pumped WMRL stream 220 at a pressure comparable to that of the desuperheated high pressure WMR stream 224. The pumped WMRL stream 220 and the desuperheated high pressure WMR stream 224 are mixed to produce mixed high pressure WMR stream 225 that is cooled in high pressure WMR condenser 226 to produce compressed WMR stream 232. The mixed high pressure WMR stream 225 is two-phase with a vapor fraction of about 0.5.
The high pressure WMR condenser 226 may be a plate and fin heat exchanger or brazed aluminum heat exchanger and must be designed to handle two-phase inlet flow. One of the challenges in doing so is that the liquid and vapor phases will distribute unevenly in the high pressure WMR condenser 226. As a result, the compressed WMR stream 232 will likely not be fully condensed, which will in turn imply reduced process efficiency for the precooling and liquefaction processes. Additionally, the two entry heat exchanger may involve operational challenges.
One approach to address these problems is to compensate for the mal-distribution of liquid and vapor in the design of high pressure WMR condenser 226 and design it to be significantly larger than in the case without mal-distribution, such that the compressed WMR stream 232 is fully condensed. However, there are two drawbacks associated with this method. First, since the degree of mal-distribution in the condenser is unpredictable, this method is somewhat arbitrary and may result in non-zero vapor fraction in compressed WMR stream 232. Second, this method results in increased capital cost and plot space, which is undesirable.
Another solution to address the problem is to cool the WMRL stream 218 and the compressed WMR stream 232 in separate tube circuits of the precooling heat exchanger 260 to about the same precooling temperature. Each cooled stream would be letdown in pressure across separate expansion devices (similar to the first WMR expansion device 237) and sent as shellside refrigerant into the precooling heat exchanger 260. Alternatively, both cooled streams could be combined and letdown in pressure in a common expansion device. This approach eliminates the issue of two-phase entry in the high pressure WMR condenser 226, however it reduces the overall efficiency of the liquefaction process, in some cases up to 4% lower efficiency as compared to FIG. 2. Further, this solution would imply an additional tube circuit in the coil wound heat exchanger or additional passages in a plate and fin heat exchanger which imply increased capital cost.
Another solution involves fully condensing the desuperheated high pressure WMR stream 224 prior to mixing with the pumped WMRL stream 220. This method further involves cooling the mixed streams in a tube circuit of the precooling heat exchanger 260. However, this method has the same drawbacks as described for the previous solution with separate tube circuits.
A further solution involves dividing the precooling heat exchanger 260 into two sections, a warm section and a cold section. In case of a coil wound heat exchanger, the warm and cold sections may be separate tube bundles within the precooling heat exchanger 260. The WMRL stream 218 is cooled in a separate tube circuit in the warm section of precooling heat exchanger 260, reduced in pressure across an expansion device, and returned as shell side refrigerant to provide refrigeration to the warm section. The compressed WMR stream 232 is cooled in a separate tube circuit in the warm and cold sections of the precooling heat exchanger 260, reduced in pressure across an expansion device, and returned as shell side refrigerant to provide refrigeration to the cold and warm sections. This arrangement eliminates the issues of two phase entry and also improve the overall efficiency of the liquefaction process as compared to FIG. 2. However, they result in significant increase in capital cost due to breaking up the precooling heat exchanger into multiple sections, and is often not desirable.
A reliable and efficient solution is desired that eliminates two-phase entry in the condenser, at the same time does not increase the capital cost of the facility significantly. This invention provides novel WMR configurations that eliminate two-phase inlet into the high pressure WMR condenser 226 as well as eliminates the WMR pump 268, thereby reducing capital cost and improving operability and design of the DMR process. The inventions may also be applied to any cooling, liquefaction or subcooling processes involving multiple component refrigerants.